1. Field of the Invention
The present invention relates to a variable focal length lens and, more particularly, to an improvement in a lens, a focal length of which can be changed at high speed in response to an electrical signal
2. Description of the Prior Art
A focal length in a conventional variable focal length optical lens system, called a "zoom lens", is changed by moving a plurality of lens groups each constituted by a plurality of single lenses, thus changing distances between the lens groups. In such a lens system, however, a moving mechanism is required to move the lens groups. The conventional lens system does not sufficiently satisfy desired high-speed change in focal length, compactness, and low cost requirement. Demand has arisen for a new and improved lens system which gives better performance.
Variable focal length lenses which solve the above problems and substantially eliminate moving parts are proposed by the present applicant in Japanese Patent Application Laid-open Nos. 157213/1982, 118618/1983, and 10224/1985, and in U.S. Pat. No. 3,520,592, issued to K. G. Leib et al. U.S. Pat. No. 3,520,592 and Japanese Patent Application Laid-open No. 10224/1985 describe simple variable focal length lenses which achieve highspeed operation. More particularly, such a variable focal length lens has a solid-state lens with optical anisotropy and a means for changing a polarization direction of light incident on the lens.
FIG. 1 is a schematic view showing a variable focal length lens proposed by Japanese Patent Disclosure No. 10224/1985.
Referring to FIG. 1, the variable focal length lens has a polarizing plate 1, a polarization plane rotating element 2, a birefringent lens 3, a power source 4, and a switch 5.
The polarization plane rotating element 2 rotates a plane of polarization of light transmitted through the polarizing plate 1 upon application of an electric field. For example, the element 2 is prepared by forming transparent electrodes on two major surfaces of a Z-cut monocrystalline KH.sub.2 PO.sub.4 plate.
The birefringent lens 3 is formed such that its optical axis (referred to as a Z-axis hereinafter) of crystal is perpendicular to the principal axis of the lens, and parallel to the surface of the drawing of FIG. 1. A refractive index of the birefringent lens 3 along a polarization direction perpendicular to the principal axis of the lens and parallel to the surface of the drawing is given as an ordinary ray refractive index n0, and a refractive index thereof along a polarization direction perpendicular to the principal axis of the lens and to the surface of the drawing is given as an extraordinary ray refractive index ne.
More specifically, referring to FIG. 1, light linearly polarized by the polarizing plate 1 is incident on the birefringent lens 3 without changing the polarization direction when the switch 5 is open. The light passing through the birefringent lens 3 is refracted at the refractive index n0, and thus has a focal length f1. In this case, the transmitted light has the same polarization direction as that of the incident light, i.e., a direction parallel to the surface of the drawing. However, when the switch 5 is closed, the polarization plane of the transmitted light is rotated 90.degree., by the polarization plane rotating element 2, relative to that of the incident light, i.e., the former coincides with a direction perpendicular to the surface of the drawing. Light passing through the birefringent lens 3 thus has a focal length f2, determined by the refractive index ne.
The conventional variable focal length lens for changing the polarization direction of light has an advantage in that the focal length can be changed within a wide range.
FIG. 2 is a schematic view of a optical information readout apparatus which adopts a variable focal length lens. The apparatus in FIG. 2 has a variable focal length lens 8.
A laser beam from a semiconductor laser 6 is focused on an optical information recording plate 12 through a collimator lens 7, a variable focal length lens 8, a polarized beam splitter 9, a 1/4 wavelength plate 10, and a focusing lens 11. The beam reflected by the optical information recording plate 12 reaches the polarized beam splitter 9 by passing through the focusing lens 11 and the 1/4 wavelength plate 10 again.
The reflected beam incident on the polarized beam splitter 9 passes through the 1/4 wavelength plate beam splitter 10 twice on the forward and backward paths. The polarization direction is changed from the polarization direction of the beam incident on the polarized beam splitter 9 to a direction perpendicular thereto. The reflected beam is thus reflected by a multilayer film formed on a diagonal line of the polarized beam splitter 9. The twice-reflected beam is focused on a photodetector 14 through a detection lens 13. The photodetector 14 converts the laser beam to an electric current, in accordance with the intensity of the beam. The processor analog current signal is converted by a data processor 15 to digital data.
In an optical data readout apparatus as described above, the direction of the linearly polarized beam incident on the polarized beam splitter 9 must always be constant.
However, when the apparatus adopts the conventional variable focal length lens shown in FIG. 1, the linearly polarized beam exit-direction is changed every time the focusing length is changed. As a result, the direction of the linearly polarized beam incident on the polarized beam splitter 9 cannot always be kept constant.
In a apparatus using a linearly polarized beam such as a laser beam, and exemplified by but not limited to the optical data readout apparatus, at least one optical element associated with the plane of polarization, such as a .lambda./4 plate or a polarized beam splitter, must often be used in consideration of specifications and apparatus configuration. In a conventional variable focal length lens, a desired focal length can be selected at high speed. However, such a lens is not compatible with all other optical apparatuses.